This is the calibre forum. We will be glad to answer calibre specific questions but for epub validation questions you might do better in the Epub forum.

This is a 'line splitting' question' caused by incorrectly applying a H# tag to a file to attempt to get Calibre to build a TOC on conversion that does not contain the normal 'Chapter'... trigger, Keywords. (Personally, I use Sigil and skip this conversion step, dance )

This is where this topic crosses over to the EPUB section

The message say the user typed <H1> in the middle of a normal paragraph (just like it appears here).

H# tags shall only exist outside of a Paragraph block. The problem may also be, there is not a matching closing tag (</H1>) within the same block.

The Open Office [insert Plug for both here ] plug-in 'Writer2EPUB' works a treat for making EPUB from well formatted DOC files.